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Whales originated from aquatic artiodactyls in the Eocene Epoch of India


Abstract and Figures

Although the first ten million years of whale evolution are documented by a remarkable series of fossil skeletons, the link to the ancestor of cetaceans has been missing. It was known that whales are related to even-toed ungulates (artiodactyls), but until now no artiodactyls were morphologically close to early whales. Here we show that the Eocene south Asian raoellid artiodactyls are the sister group to whales. The raoellid Indohyus is similar to whales, and unlike other artiodactyls, in the structure of its ears and premolars, in the density of its limb bones and in the stable-oxygen-isotope composition of its teeth. We also show that a major dietary change occurred during the transition from artiodactyls to whales and that raoellids were aquatic waders. This indicates that aquatic life in this lineage occurred before the origin of the order Cetacea.
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Whales originated from aquatic
artiodactyls in the Eocene epoch of India
J. G. M. Thewissen
, Lisa Noelle Cooper
, Mark T. Clementz
, Sunil Bajpai
& B. N. Tiwari
Although the first ten million years of whale evolution are documented by a remarkable series of fossil skeletons, the link to
the ancestor of cetaceans has been missing. It was known that whales are related to even-toed ungulates (artiodactyls), but
until now no artiodactyls were morphologically close to early whales. Here we show that the Eocene south Asian raoellid
artiodactyls are the sister group to whales. The raoellid Indohyus is similar to whales, and unlike other artiodactyls, in the
structure of its ears and premolars, in the density of its limb bones and in the stable-oxygen-isotope composition of its teeth.
We also show that a major dietary change occurred during the transition from artiodactyls to whales and that raoellids were
aquatic waders. This indicates that aquatic life in this lineage occurred before the origin of the order Cetacea.
Phylogenetic analyses of molecular data on extant animals strongly
support the notion that hippopotamids are the closest relatives of
cetaceans (whales, dolphins and porpoises)
. In spite of this, it is
unlikely that the two groups are closely related when extant and
extinct artiodactyls are analysed, for the simple reason that cetaceans
originated about 50 million years (Myr) ago in south Asia, whereas
the family Hippopotamidae is only 15 Myr old, and the first hippo-
potamids to be recorded in Asia are only 6 Myr old
. However, ana-
lyses of fossil clades have not resolved the issue of cetacean relations.
Proposed sister groups ranged from the entire artiodactyl order
the extinct early ungulates mesonychians
, to an anthracotheroid
(which included hippopotamids), to weakly supporting hip-
popotamids (to the exclusion of anthracotheres
The middle Eocene artiodactyl family Raoellidae
is broadly
coeval wit h the earliest cetaceans, and both are endemic to south
Asia. Raoellids, as a composite consisting of several genera, have been
added to some phylogenetic analyses
, but no close relation to
whales was found because raoellid fossils were essentially limited to
dental material
. We studied new dental, cranial and postcranial
material for Indohyus, a middle Eocene raoellid artiodactyl from
Kashmir, India (Fig. 1). All fossils of Indohyus were collected at a
middle Eocene bone bed extending for about 50 m at the locality
Sindkhatudi in the Kalakot region of Kashmir on the Indian side of
the Line of Control. Our analysis identifies raoellids as the sister
group to cetaceans and bridges the morphological divide that sepa-
rated early cetaceans from artiodacyls. This has profound implica-
tions for the character transformations near the origin of cetaceans
and the cladistic definition of Cetacea, and identifies the habitat in
which whales originated. Taken together, our findings lead us to
propose a new hypothesis for the origin of whales.
Cetaceans and raoellids are sister groups
To investigate the importance of raoellids in cetacean phylogeny, we
excluded raoellids from our initial phylogenetic analysis of artiodac-
tyls plus cetaceans. Our data set differed from previous analyses
the addition of several archaic anthracotheres, and some corrected
scores for pakicetid cetaceans. This analysis found stronger support
for hippopotamid–cetacean sister-group relations than the previous
, consiste nt with molecular studies
. However, the base of
the artiodactyl cladogram is poorly resolved (see Supplementary
Information for details on phylogenetic runs). In a second cladistic
analysis (Fig. 2), we added the raoellids Khirtharia and Indohyus as
well as several archaic ungulate groups (condylarths) and found that
raoellids and cetaceans are sister groups and that they are the basal
node in the Cetacea/Artiodactyla clade, consistent with some pre-
vious analyses that used different character sets
. Our analysis is the
first to show that raoellids are the sister group to cetaceans, resolving
the biogeographic conundrum and closing the temporal gap between
cetaceans and their sister. Relations between most artiodactyl families
higher in the tree are poorly resolved, and our data lack implications
for the relations between these families. Our analysis strongly argues
that raoellids and cetaceans are more closely related to each other
than either is to hippopotamids.
Indohyus shares with cetaceans several synapomorphies that are
not present in other artiodactyls. Most significantly, Indohyus has a
thickened medial lip of its auditory bulla, the involucrum (Figs 1 and
3), a feature previously thought to be present exclusively in cetaceans.
Involucrum size varies among cetaceans, but the relative thickness of
medial and lateral walls of the tympanic of Indohyus is clearly within
the range of that of cetaceans and is well outside the range of other
cetartiodactyls (Fig. 3). Other significant derived similarities between
Indohyus and cetaceans include the anteroposterior arrangement of
incisors in the jaw, and the high crowns in the posterior premolars.
Characterizing Cetacea
Until now, the involucrum was the only character occurring in
all fossil and recent cetaceans but in no other mammals
Identification of the involucrum in Indohyus calls into question
what it is to be a cetacean: it requires either that the concept of
Cetacea be expanded to include Indohyus or that the involucrum
cease to characterize cetaceans. We argue that the content
of Cetacea should remain stable and include Pakicetidae,
Ambulocetidae, Remingtonocetidae, Protocetidae, Basilosauridae,
Mysticeti and Odontoceti
. Thus, Cetacea remains a mono-
phyletic group, whereas Artiodactyla remains a paraphyletic group
(because Raoellidae are included but Cetacea are excluded).
An alternative classification would render both Cetacea and
Artiodactyla monophyletic by including Raoellidae in Cetacea and
Department of Anatomy, Northeastern Ohio Universities College of Medicine, Rootstown, Ohio 44272, USA.
School of Biomed ical Sciences, Kent State University, Kent, Ohio 44242,
Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA.
Department of Earth Sciences, Indian Institute of Technology, Roorkee,
Uttarakhand 247 667, India.
Wadia Institute of Himalayan Geology, Dehra Dun, Uttarakhand 248 001, India.
Vol 450
20/27 December 2007
by limiting Artiodactyla to those clades one node above the raoellid/
cetacean node (Fig. 2). We do not prefer this classification because it
causes instability by significantly altering the traditional content of
both Artiodactyla and Cetacea.
Characters identified as synapomorphies for Cetacea in some of
our most parsimonious trees include: long external auditory meatus,
double-rooted P3/, lack of P4/ protocone, M1-2/ metacones present
but small, and lack of M1-2/ hypocone. None of these features char-
acterize all modern and extinct cetaceans; the dental characters, for
instance, cannot be scored in toothless mysticetes. In addition, all of
these characters are found in some mammals unrelated to cetaceans.
We attach particular importance to two character complexes that
characterize basal cetaceans, constitute synapomorphic suites for the
order, and are of great functional importance. All fossil and recent
cetaceans differ from most other mammals in the reduction of crush-
ing basins on their teeth: there are no trigonid and talonid basins in
the lower molars, and the trigon basin of the upper molars is very
small (for example in pakicetids and ambulocetids) or absent.
Crushing basins are large in raoellids (Fig. 1a, b) and other basal
ungulates. This implies that a major change in dental function
occurred at the origin of cetaceans, probably related to dietary change
at the origin
. Reduced crushing basins also occur in mesonychids,
archaic ungulates long thought to be closely related to cetaceans.
However, mesonychian molars have wear facets very unlike those
of cetaceans
, whereas wear facets in raoellids are more similar to
wear facets in early cetaceans
The second character complex that identifies cetaceans is the shape of
the postorbital and temporal region of the skull. In early cetaceans, this
region is long and narrow
. This affects the sense organs: the olfactory
peduncle is long and narrow and the orbits are set close together near
the roof of the skull. It also affects oral function, the nasopharyngeal
duct is narrow, and the out-lever of the masticatory muscles is long,
increasing the closing speed of the jaws. We speculate that the import-
ance of different sense organs was related to these changes, or that
changing diet led to a change in food-processing organs.
1 cm
1 cm
Middle ear
Tympanic bulla
Figure 1
Osteology of Indohyus and cross-sections of long bones of
Eocene cetartiodactyls. a
, b, Oblique lateral view of skull RR 208 (a) and
ventral view of skull RR 207 (
b). ch, Posterior views of humerus (RR 149,
c) and femur (RR 101, d), plantar views of metacarpal (RR 138, e) and
proximal manual phalanx (RR 19,
f), dorsal view of astragalus (RR 224, g),
and posterior view of metatarsal (RR 139,
h). il, Histological mid-shaft
sections for humerus of the pakicetid Ichthyolestes (H-GSP 96227,
humerus of Indohyus (RR 157,
j), femur of Indohyus (RR 42, k) and femur of
the artiodactyl Cainotherium (IVAU unnum,
l). Both scale bars are 1 cm; the
scale bar near
d goes with ah, and that near l goes with il.
Pachyaena gigantea
Pachyaena ossifraga
r Artiodactyl
Cetacea Raoellidae
Figure 2
Phylogeny of artiodactyls, cetaceans and archaic ungulates. The
figure shows a consensus cladogram produced by heuristic searches with
PAUP (random addition sequence, 1,000 repetitions), using a published
data set
. See Supplementary Information for further details.
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20/27 December 2007 ARTICLES
Indohyus was aquatic
Behaviourally, the earliest whales (pakicetids) were aquatic
. This led us to investigate whether Indohyus was aquatic
too. Cortical bone thickness in secondarily aquatic tetrapods is com-
monly increased at the expense of the medullary cavity, a pattern
called osteosclerosis
. Osteosclerosis occurs in early whales
, sea otters
, Hippopotamus
, beavers
, pinnipeds
and Mesozoic marine reptiles
. Osteosclerosis provides ballast
that allows some aquatic taxa to be bottom walkers (hippopotamids)
and others to maintain neutral buoyancy in water (manatees)
Histological sections indicate that the limb bones of Indohyus are
also osteosclerotic (Fig. 1i–l), in a similar manner to those of paki-
cetid cetaceans. Our survey of cortical bone thickness in the limb
bones of terrestrial artiodactyls shows that this pattern is unusual
for that order: in mid-bone cross-sections of the femur, the medul-
lary cavity makes up between 0.60 and 0.75 of the width of the bone,
whereas in aquatic mammals the values are lower (Hippopotamus,
0.55; pakicetids and ambulocetids, 0.25–0.57). In Indohyus this ratio
is 0.42, suggesting that Indohyus was osteosclerotic and thus aquatic.
We interpret the limb osteosclerosis of Indohyus to be related to
bottom walking and not to slow swimming, because the limbs are
gracile and not modified into paddles.
To investigate further the hypothesis that Indohyus was aquatic, we
studied the stable isotopes of its enamel, a tissue relatively resistant
to preburial and postburial alteration of isotopic composition
Enamel d
O values are influenced by the oxygen isotope composi-
tion of the food and water ingested by an animal as well as by certain
physiological processes (such as sweating, panting and respiration)
For aquatic species, the flux of environmental water by means of
direct ingestion and transcutaneous exchange overwhelms all other
oxygen sources
and can cause the enamel d
O values of freshwater
taxa (for example Hippopotamus) to be 2–3% lower than those for
terrestrial mammals
. Mean d
O values for four individuals of
Indohyus are at least 2% lower than those for our comparative sample
of Eocene terrestrial and semi-aquatic mammals from formations of
India and Pakistan of similar or slightly older age (Fig. 4)
Although not representative of the specific deposits from which
Indohyus was collected, oxygen isotope values for each ecological type
from these sites (namely terrestrial, 24–28%; semi-aquatic, 23% or
less) are surprisingly consistent regardless of age or location (Fig. 4).
This suggests that temporal and spatial variation in environmental
isotope values was relatively minor and was most probably insuf-
ficient to account for the extremely low d
O we have reported for
Indohyus. We did not recover tooth material of other mammals at the
Indohyus site; until such material can be analysed, the most consistent
interpretation is that these low values are a result of the aquatic habits
of this species.
Supporting evidence for a semi-aquatic life of Indohyus comes
from examination of its inter-individual variation in d
O values.
The overwhelming influx and mitigating influence of isotopically
homogeneous environmental water causes the variation in individual
O values for semi-aquatic and aquatic species (s.d. , 0.5%)tobe
much lower than that of terrestrial species (s.d. . 1.0%)
. This line
of evidence is especially relevant in our study because it does not
require an approximation of the mean environmental isotope values
for a site through analysis of the associated fauna. Variation in d
values for Indohyus (s.d. 5 0.4%) is extremely low and when com-
pared with species of sufficient sample size (n . 3) it is similar to that
of the semi-aquatic archaeocete Pakicetus. Given that the influence of
physiological and environmental factors on body water d
O values is
more strongly felt at smaller body sizes
, this low level of variation is
particularly compelling for Indohyus, with a body mass of less than
50 kg.
To explore the diet of Indohyus we studied carbon isotopes.
Enamel d
C values are defined by the carbon isotope composition
of an animal’s diet and can be used to identify the food webs and
resources used by an animal
. The d
C values of primary producers
at the base of aquatic and terrestrial food webs overlap, but values for
freshwater phytoplankton are typically depleted in
C relative to
freshwater macrophytes
, and both types of aquatic producer are
depleted in
C relative to terrestrial C
. Consumers for-
aging within food webs fuelled by freshwater phytoplankton (for
example freshwater and brackish-water foraging Eocene whales) typ-
ically have lower d
C values than species foraging on aquatic macro-
or on terrestrial resources
(Fig. 4). Enamel d
C values for
Indohyus are higher than those for most early cetaceans and are most
similar to the d
C values in enamel for terrestrial mammals from
early and middle Eocene deposits in India and Pakistan. Indohyus
could have been feeding on land or in water, but it was clearly eating
something different from archaeocetes such as Pakicetus and
Ambulocetus. If the large crushing basins in the molars of Indohyus
were used for processing vegetation, these d
C values in enamel
could come from the ingestion of terrestrial plants or aquatic macro-
phytes. Alternatively, a more ominivorous diet would suggest that
Indohyus might have foraged on benthic, aquatic invertebrates in
5.0 5.5
n [Width
ital co
Tympanic thickness (medial/lateral walls)
Figure 3
Plot of the ratio of the thickness of the medial tympanic wall to
that of the lateral tympanic wall against the natural logarithm of the width
across occipital condyles, showing that the ratio in Indohyus is similar to
that in cetaceans.
In cetaceans (open squares), the medial tympanic wall is
inflated and called the involucrum, and the lateral tympanic wall is thinned
and called the tympanic plate. In artiodactyls (open triangles), the medial
and lateral tympanic walls are more similar in thickness, causing values on
the y axis to be closer to 1. See Supplementary Information for further
20 22 24 26 28 30
Freshwater and
Anthracobunids and
Figure 4
Bivariate plot of d
O and d
C values for enamel samples of
early and middle Eocene mammals from India and Pakistan.
Results are
shown as means 6 s.d. for the sample population. Filled triangle, Indohyus;
open triangle, Khirtharia; open squares, terrestrial mammals; filled squares,
brackish-water anthracobunids; filled circles, freshwater/brackish-water
archaeocetes. See Supplementary Information for details.
Vol 450
20/27 December 2007
freshwater systems. Although we cannot exclude the possibility of
aquatic foraging by Indohyus, d
C values in enamel do suggest that
the diet of Indohyus differed significantly from that of Eocene whales.
A more refined interpretation of the dietary preferences of Indohyus
will require a study of tooth wear and tooth morphology.
Evolutionary hypothesis for whale origins
Indohyus was a small, stocky artiodactyl, roughly the size of the rac-
coon Procyon lotor (Fig. 5). It was not an adept swimmer; instead it
waded in shallow water, with its heavy bones providing ballast to keep
its feet anchored. Indohyus may have fed on land, although a special-
ized aquatic diet is also possible.
The modern artiodactyl morphologically most similar to Indohyus
is probably the African mousedeer Hyemoschus aquaticus.
Hyemoschus lives near streams and feeds on land, but flees into the
water when danger occurs
. Indohyus had more pronounced aquatic
specializations than Hyemoschus does, and it probably spent a con-
siderably greater amount of time in the water either for protection or
when feeding. As indicated by the evidence from stable isotopes,
Indohyus spent most of its time in the water and either came onshore
to feed on vegetation (as the modern Hippopotamus does) or foraged
on invertebrates or aquatic vegetation in the same way that the mod-
ern muskrat Ondatra does.
Raoellids are the sister group to cetaceans, and this implies that
aquatic habitats originated before the Order Cetacea. The great
evolutionary change that occurred at the origin of cetaceans is thus
not the adoption of an aquatic lifestyle. Here we propose that dietary
change was the event that defined cetacean origins; this is consistent
with the cranial and dental synapomorphies identified. Molars of
Indohyus are markedly different from those of pakicetids, and it is
widely assumed that pakicetids ate aquatic prey
Our working hypothesis for the origin of whales is that raoellid
ancestors, although herbivores or omnivores on land, took to fresh
water in times of danger. Aquatic habits were increased in Indohyus
(as suggested by osteosclerosis and oxygen isotopes), although it did
not necessarily have an aquatic diet (as suggested by carbon isotopes).
Cetaceans originated from an Indohyus-like ancestor and switched to
a diet of aquatic prey. Significant changes in the morphology of the
teeth, the oral skeleton and the sense organs made cetaceans different
from their ancestors and unique among mammals.
We chose an existing character matrix
as the basis for our phylogenetic analysis.
We corrected scores of some of the taxa, and made some changes in the taxa
included. Details on these taxa, the rationale for using them, and their scores are
given in Methods and in Supplementary Table 1.
Tympanic wall thickness was investigated to address the presence of the
involucrum quantitatively. We calculated the ratio of medial tympanic wall
thickness divided by lateral tympanic wall thickness (see Methods and
Supplementary Table 2). Bone histology was studied to investigate the presence
of osteosclerosis. It was quantified as the ratio of medullary cavity width
divided by bone width in the mediolateral plane (see Methods and
Supplementary Table 3).
For analysis of stable isotopes, we prepared powders by following published
methods (see Methods and Supplementary Tables 4 and 5). Multiple samples
were collected for each species to provide an estimate of population means for
carbon and oxygen isotope values
. About 5 mg of enamel powder was collected
from each specimen (tooth) for study of carbon and oxygen isotope values.
Most fossils of Indohyus were collected by the late Indian geologist A. Ranga
Rao, who discovered the locality about 25 years ago (acronym RR); additional
fossils were collected by S.B. and B.N.T. at the same locality (acronym IITR-SB-
Full Methods and any associated references are available in the online version of
the paper at
Received 26 June; accepted 3 October 2007.
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C values in leaves of aquatic macrophytes from different habitats in Britain
and Finland; some implications for photosynthetic processes in aquatic plants.
Oecologia 50, 117
124 (1981).
39. O’Leary, M. H. Carbon isotopes and photosynthesis. Bioscience 38, 328
40. Dubost, G. Un aperc¸u sur l’e
cologie du chevrotain africain Hyemoschus aquaticus
Ogilby, artiodactyle tragulide. Mammalia 42, 1
62 (1978).
Supplementary Information is linked to the online version of the paper at
Acknowledgements We thank the late F. Obergfell for presenting us with the
sediment blocks containing Indohyus fossils collected by A. Ranga Rao for
preparation and study; D. S. N. Raju and N. Raju for facilitating our research;
B. Armfield, R. Conley and A. Maas for fossil preparation; J. Dillard for preparing
Fig. 5; and J. Geisler and J. Theodor for providing additional information about their
cladistic analyses. Laboratory research was funded by the National Science
Foundation (NSF)
Earth Sciences (grants to J.G.M.T. and M.T.C.). Collaborative
work was funded by the Indian Department of Science and Technology (to S.B.)
and the NSF
International Division (to J.G.M.T.) under the Indo-US Scientific
Cooperation Program. Laboratory analyses were supported by the Skeletal Biology
Research Focus Area of Northeastern Ohio Universities College of Medicine.
Author Contributions J.G.M.T. was responsible for anatomical and systematic
study, and scientific synthesis, L.N.C. for systematic and bone density study,
M.T.C. for the study of stable isotopes, and S.B. and B.N.T. for geological study and
collecting of Indohyus and comparative fossil samples.
Author Information Reprints and permissions information is available at Correspondence and requests for materials should be
addressed to J.G.M.T. (
Vol 450
20/27 December 2007
Systematic study. We chose an existing, published character matrix
as the basis
for our phylogenetic analysis because this matrix is rich in characters and con-
tains most relevant taxa. From this matrix we deleted those artiodactyls and
cetaceans that are geologically young or modern and are well represented by
fossil relatives (Odocoileus, Bos, Ovis, Remingtonocetus, Protocetus, Georgiacetus,
Basilosaurus, Balaenoptera, Physeter, Tursiops, Delphinapterus, Camelus and
Lama), as well as perissodactyls and non-ungulate taxa.
To this matrix, we added the anthracotheres Siamotherium, Anthracokeryx and
Microbunodon, because they are near the base of the anthracotheroid clade
(Anthracotheriidae plus Hippopotamidae) and are sometimes thought to be
close to early whales
. Scores for these taxa, and the sources on which we based
the scores, are listed in Supplementary Table 1 (refs 4, 6, 41–46).
We chose Gujaratia pakistanensis (formerly Diacodexis pakistanensis)
as out-
group for the analyses of cetaceans plus artiodactyls, and we chose Arctocyon and
Hyopsodus as outgroups for the (second) analysis that included all taxa (Fig. 2).
We corrected some of the scores for Pakicetidae
, because new fossils have
been published for this family, in particular cranial material
and postcranial
. Corrected scores for pakicetids are also listed in Supplementary Table
1 and were based on original material in the Howard-Geological Survey of
Pakistan (H-GSP) collections, currently curated by J.G.M.T.
Raoellidae have been included in several previous phylogenetic analyses relat-
ing to early whales
. These authors based raoellid scores on Khirtharia and
Indohyus. In the present analyses we have split scores for these animals, with
Khirtharia scores based mostly on published H-GSP material and one unpub-
lished skull (H-GSP 1979; dentition published
, specimen now lost). Scores for
Indohyus are based on the material in the RR and IITR-SB collections; all raoellid
scores are listed, with the specimen number of the fossil on which the score was
based, in Supplementary Table 1.
Study of tympanic walls. Tympanic wall thickness was investigated to address
the presence of the involucrum quantitatively. The involucrum is the thickened
medial wall of the tympanic bone (the ossified wall of the middle ear cavity). The
lateral tympanic wall of cetaceans is reduced in thickness (the tympanic plate).
To quantify these differences in tympanic walls, we calculated the ratio of medial
tympanic wall thickness to the lateral tympanic wall thickness. Lateral tympanic
wall thickness was measured with a micrometer (Dyer gauge) just inferior to
the tympanic ring, and medial tympanic wall thickness was measured directly
across from this site on the other (medial) side of the middle ear cavity (see
Supplementary Table 2).
Bone histology. Bone histology was studied to investigate the presence of osteo-
sclerosis. Osteosclerosis is the thickening of the cortical bone. It was quantified as
the ratio of medullary cavity width divided by bone width in the mediolateral
plane, because left and right cortical thickness plus medullary cavity thickness
equals bone width (see Supplementary Table 3). Measurements were taken on
the femur with callipers.
Fossil limb shaft fragments were embedded in Buehler low-viscosity epoxy
resin and sectioned with a diamond saw. Sections were mounted on frosted glass
slides by using epoxy resin. Mounted sections were then ground down and
polished to a thickness of about 75 mm by using a precision grinder with 600,
800 and 1,200 grit paper
(Fig. 1i–l).
Study of stable isotopes. For the analysis of stable isotopes, three or more speci-
mens of each species were analysed (when available; see Supplementary Tables 4
and 5) to provide a robust estimate of the population mean and s.d. for carbon
and oxygen isotope values
. About 5 mg of enamel powder was collected from
each specimen, either by drilling directly from the tooth or by grinding enamel
chips in an agate mortar and pestle. Before collection, contaminants were
removed by abrading the outer surface of the specimen.
Preparation of powders for analysis of stable isotopes followed published
. Powders were first transferred to 1-ml microcentrifuge vials and then
soaked sequentially overnight in about 0.20 ml of a sodium hypochlorite solu-
tion (1–2 g dl
) and then in about 0.20 ml of calcium acetate buffered acetic acid
(pH about 5.1). On addition of each reagent, samples were agitated for 1 min on
a Vortex Genie vortex mixer. After each soak, the supernatant was removed by
aspiration and the residual powder was rinsed five times with deionized water.
Samples were then freeze-dried overnight and about 1.5 mg of powder from each
was weighed into individual test tubes for analysis on a Thermo-Finnigan gas
bench autosampler attached to a Thermo Finnigan Delta
XP continuous-flow
isotope-ratio mass spectrometer at the University of Wyoming Stable Isotope
All values for stable isotopes are reported in delta (d) notation, using the
equation d(%) 5 1,000 3 (R
2 1), where R
is the observed
isotope ratio of the sample (
O) and R
is the accepted
ratio for an appropriate international standard (Vienna Pee Dee belemnite for
C; Vienna Standard Mean Ocean Water for d
O). Analytical precision is
typically better than 0.1% for d
C values and 0.2% for d
O values (61s).
41. Ducrocq, S. The late Eocene Anthracotheriidae (Mammalia, Artiodactyla) from
Thailand. Palaeontographica A 252, 93
140 (1999).
42. Ducrocq, S. Unusual dental morphologies in late Eocene anthracotheres
(Artiodactyla, Mammalia) from Thailand: dental anomalies and extreme
variations. N. Jb. Geol. Pala
ont. MH 4, 199
212 (1999).
43. Suteethorn, V., Buffetaut, E., Helmcke-Ingavat, R., Jaeger, J. J. &
Jongkanjanasoontorn, Y. Oldest known Tertiary mammals from South East Asia:
middle Eocene primate and anthracotheres from Thailand. N. Jb. Geol. Pala
ont. MH
9, 563
570 (1988).
44. Colbert, E. H. Fossil mammals from Burma. Am. Mus. Nat. Hist. Bull. 74, 419
45. Brunet, M. M. De
couverte d’un cra
ne d’Anthracotheriidae, Microbunodon
minimum (Cuvier), a
La Milloque (Lot-et-Garonne). C. R. Acad. Sci. Paris 267,
838 (1968).
46. Lihoreau, F., Blondel, C., Barry, J. & Brunet, M. A new species of the genus
Microbunodon (Anthracotheriidae, Artiodactyla) from the Miocene of Pakistan:
genus revision, phylogenetic relationships and palaeobiogeography. Zool. Scr. 33,
115 (2004).
47. Bajpai, S. et al. Early Eocene land mammals from Vastan Lignite Mine, District
Surat, Gujarat, western India. J. Palaeontol. Soc. India 50, 101
113 (2005).
48. West, R. M. Middle Eocene large mammal assemblage with Tethyan affinities,
Ganda Kas region, Pakistan. J. Paleontol. 54, 508
533 (1980).
... To translate 42 the microevolutionary phenomenon into macroevolutionary 'rule', [5] attempted to 43 recognize it analyzing 854 estimates from contemporaneous natural populations. For size, 44 79% of selection estimates exceed zero, whereas for other morphological traits positive 45 and negative values are similar in frequency. The predominance of positive directional 46 selection on size within populations could translate into macroevolutionary trend toward 47 increased size and thereby may explain Cope's Rule. ...
... This phe-98 nomenon and the rapid size increase of Mysticeti documented by the fossil record but are 99 not fully understood yet; however, [17] suggested that there has been a selection for low-100 frequency sounds (enabling, e.g., communication over long distances). [44] demonstrated 101 that Cetacea (and extinct raoellids) are sister groups of artiodactyls (including the extant 102 hippos) and Cetacea is a monophyletic group. The significant changes of the teeth, the 103 oral skeleton and the sense organs made cetaceans different from their ancestors and 104 unique among mammals. ...
Tendency of the vertebrates to increase body sizes during evolution is recognized in the 19th century. Beside the contradictory Cope’s rule, other ‘rules’ were recognized, however not fully explained the evolutionary size growth phenomenon. Recently the resource rule offered a plausible explanation to this, although it left open the rapid evolutionary size growth of the Mysticeti (Mammalia, Vertebrata). Here, the rapid and significant evolutionary size growth of Mysticeti and the odontocete sperm whale is explained by the innovative application of the long-range communication channels. Regarding all analysed extant species using infrasound either in aquatic or terrestrial environs for long-range communication, a correlation between the body size/mass and maximum propagation of the applied infrasound is recognized. Correlation of the body sizes of these infrasound generating vertebrate taxa with the maximum range of audibility reflects the acoustical characteristics of the applied long-range communication channels. The wider the audibility in the communication channel is, the greater the body size of the species using the SOFAR channel for long-range communication. Other, non-aquatic, terrestrial vertebrate species (African elephants, cassowary) also present the same phenomenon suggesting that application of the long-range communication channels may have universal role in evolutionary size growth among social animals.
... The similar bilophodont upper molars in Raoellidae and Tapirulus is therefore attributed to the convergence. Raoellidae has recently been considered to be the sister to Cetacea (Thewissen et al., 2007;Cooper et al., 2012;Thewissen et al., 2020), and distantly related to Tapirulus. ...
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Artiodactyls diversified during the Eocene and Oligocene in North America and Europe after their first Holarctic appearance at the beginning of the Eocene. However, the relationships among early artiodactyls, European endemic forms, and later derived suiforms, tylopods, and ruminants remain unclear. Early artiodactyls are relatively rare in Asia compared to those known from North America and Europe; thus, investigation of Eocene artiodactyls from Asia is important to resolve these issues. Here we report two new genera and three new species of small early artiodactyls from middle Eocene deposits of the Erlian Basin, Inner Mongolia, China. The new materials represent a morphologically gradational series from Asian Land Mammal Ages Irdinmanhan to Sharamurunian, characterized by a trend towards bilophodonty in the lower molars. Morphologic and phylogenetic analyses suggest that these new taxa have a close relationship with the enigmatic European Tapirulus, which currently consists of five species that range from the middle Eocene to the early Oligocene. The close relationship between the Erlian specimens and Tapirulus suggests possible faunal exchanges between Europe and Asia during the middle Eocene, a view that has been supported by other mammalian groups across the two continents. The evolution of bilophodonty in Tapirulidae and Raoellidae is probably attributable to convergence.
... Abbreviations: Ce Cetacea, EA endemic artiodactyls, Hi Hippopotamoidea, Or Oreodontoidea, Ru Ruminantia, Su Suoidea, Ty Tylopoda. Phylogenetic relationships are based on Hassanin et al. (2012) for crown groups branching, fossil taxa relationships rely on Thewissen et al. (2007) for Cetacea , Geisler et al. (2007) for Tylopoda, Métais and Vislobokava (2007) for Ruminantia, Orliac (2012) for Suoidea, Oreodontoidea and endemic European taxa are here considered as monophyletic groups, relationships within endemic European taxa rely on Weppe et al. (2020a). Not to scale molecular grounds (Hassanin et al. 2012), the position of extinct taxa is still highly disputed (e.g. ...
This chapter presents a detailed review of works published on Artiodactyla endocasts and provides a comprehensive examination of artiodactyl brain evolutionary history, including Cetacea, from the early Eocene (c.a. 45 Ma) onwards. Artiodactyl endocasts have been studied from the second half of the nineteenth century to the 1970s. These works on natural or artificial endocasts widely took place outside the frame of phylogenetic concerns. We compile the data available, including recent works using μCT-scan imagery techniques, and place them in a phylogenetic framework. We also provide new data regarding Paleogene representatives of North American extinct clades (Homacodon, Helohyus, Leptauchenia, Agriochoerus), endemic European clade (Mouillacitherium, Dichobune), and Suoidea (Palaeochoerus). The brain of modern artiodactyls is remarkable by the expansion and by the folding of the neopallium. Their brains are highly gyrencephalic and differ in their neocortical pattern. We highlight diversity of neopallium patterns of Artiodactyla and their convergent nature on the last 45 millions years and show that encephalization increases with time, but with different modes between terrestrial and fully aquatic taxa (i.e. Cetacea). Each clade shows a mosaic pattern of derived and plesiomorphic features that now has to be put in perspective with both the history and ecology of taxa.
Since molecular data identified hippopotamids as the closest living relatives of cetaceans, a common aquatic/semiaquatic ancestor hypothesis for these modern taxa has naturally been proposed. However, recent molecular studies concluded that most molecular adaptations in extant cetaceans occurred after their split from hippopotamids. If the question of aquatic affinities of the first cetaceans has been investigated at large, it has not been the case for the forebears of hippopotamids. Sensory organs are drastically affected by underwater perception. In this work, we question the aquatic affinities of fossil hippopotamoids through an investigation of the morphology and morphometrics of the petrosal bone and cochlea of 12 extinct hippopotamoid taxa. Petrosal and bony labyrinth morphological characters constitute a source of structured phylogenetic signal, both supporting major hippopotamoid clades and bringing original relationships. The morphometric study of functional measurements of the cochlear canal shows that anthracotheres bridge the morphological gap between terrestrial artiodactyls and modern hippopotamids. The integrated functional signal of the ear region further supports convergent acquisitions of semiaquatic behaviour in hippopotamids and cetaceans and indicates that terrestrial hearing was an ancestral trait among Hippopotamoidea. We highlight multiple convergent acquisitions of semiaquatic traits in the ear region of hippopotamoids and new robust phylogenetic characters.
The quintessential example of evolutionary convergence is that between the shark, ichthyosaur, and dolphin. Although not closely related, the three exemplar taxa have independently evolved adaptations in morphology, physiology, and behavior that result in concomitant levels of performance that meet the requirements associated with operating in a dense, viscous, and thermally conductive marine environment. These apex marine predators display a remarkable amount of homoplasy. All three taxa have developed streamlined fusiform bodies to reduce drag when swimming. The position, type, and morphology of the control surfaces (i.e., fins, flippers, flukes) are similar for the convergent taxa. The control surfaces have different internal support structures, but function similarly to generate lift forces for stability and maneuverability. The main departure in control surface design among the three taxa is that dolphins lack pelvic fins. For dolphins, the loss of pelvic appendages is directly related to the possession of horizontally oriented caudal flukes, which perform double duty as a propulsive device and posterior stabilizer for trim control. The flukes of dolphins and caudal fins of ichthyosaurs and sharks have a lunate shape that function as an oscillating wing to generate high efficiency, lift-based thrust for high-speed swimming. The three convergent taxa are homeothermic, with a body temperature above that of the water in which they live. The advantages of an elevated body temperature are the attainment of higher maximum swimming speeds, longer and faster sustained swimming speeds, improved digestion, brain heating, and enhanced visual acuity. The convergence of the shark, ichthyosaur, and dolphin with respect to morphology, physiology, and locomotor performance reflects similar selective pressures imposed by the physical fluid environment that have dictated the independent evolutionary trajectories of these high-performance marine predators.
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Cetaceans are a group of aquatic mammals with the largest body sizes among living animals, including giant representatives such as blue and fin whales. To understand the genetic bases of gigantism in cetaceans, we performed molecular evolutionary analyses on five genes (GHSR, IGF2, IGFBP2, IGFBP7, and EGF) from the growth hormone/insulin-like growth factor axis, and four genes (ZFAT, EGF, LCORL, and PLAG1) previously described as related to the size of species evolutionarily close to cetaceans, such as pigs, cows, and sheep. Our dataset comprised 19 species of cetaceans, seven of which are classified as giants because they exceed 10 m in length. Our results revealed signs of positive selection in genes from the growth hormone/insulin-like growth factor axis and also in those related to body increase in cetacean-related species. In addition, pseudogenization of the EGF gene was detected in the lineage of toothless cetaceans, Mysticeti. Our results suggest the action of positive selection on gigantism in genes that act both in body augmentation and in mitigating its consequences, such as cancer suppression when involved in processes such as division, migration, and cell development control.
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“(molinai), el encuentro con la naturaleza” es un libro de divulgación científica que contiene información sobre el Abate Juan Ignacio Molina González, uno de los primeros científicos y naturalistas de Chile, nacido en Villa Alegre, Región del Maule. El libro se encuentra dividido en ocho capítulos. Cada uno comienza con un relato que incluye información histórica acerca de Molina y también acontecimientos de ficción. Además, presentan infografías con contenidos sobre los ecosistemas y la biodiversidad de la zona central de Chile con el objetivo de divulgar información rigurosa de manera visual, simplificada y sencilla. Las ilustraciones incluidas en el libro son representaciones y diseños artísticos, y tienen la intención de incentivar la lectura y la imaginación, por lo que no representan necesariamente los colores y proporciones reales de las especies y los personajes. Adicionalmente, el libro cuenta con una aplicación móvil que permite proyectar las ilustraciones en dos y tres dimensiones, y también audios de los relatos en formato digital. Proyecto financiado por el Ministerio de Ciencia, Tecnología, Conocimiento e Innovación de Chile
A nearly complete skull of Remingtonocetus harudiensis was discovered from the Harudi Formation of the Kutch Basin, western India. Though several specimens have been collected over the last two decades by earlier workers, this skull shows variations in morphology within the species that were not known earlier. Hence, this finding is significant because it helps in expanding our knowledge of the skull morphology with the addition of characters such as the larger size of the skull, the difference in dental morphology, and the two-ridged external nasal feature and our understanding of how the molar morphology can vary within the same species. The newly excavated skull is the largest Remingtonocetus skull so far discovered and is similar to the size of Dalanistes ahmedi from Kutch, India and from Baluchistan, Pakistan. Thus, the range of the overall body size of the species will change considerably towards the higher side nullifying a major distinction between Dalanistes and Remingtonocetus. The new skull was excavated from the chocolate brown shales of the clastic facies of the Harudi Formation, in the inner ramp of a lagoonal phase. It is approximately 4 m above the nodular limestones from which the other Remingtonocetus materials were previously described. The limestone represents the carbonate facies of the Formation formed in a middle to outer ramp setting. An emended diagnosis of R. harudiensis is provided in this study. The Bartonian Harudi Formation of the Kutch Basin, thus, becomes unique in having Remingtonocetus specimens discovered from two different lithological facies from both the inner and outer ramp settings.
During the last century enormous progress has been made in the understanding of biological diversity, involving a dramatic shift from macroscopic to microscopic organisms. The question now arises as to whether the Natural System introduced by Carl Linnaeus, which has served as the central system for organizing biological diversity, can accommodate the great expansion of diversity that has been discovered. Important discoveries regarding biological diversity have not been fully integrated into a formal, coherent taxonomic system. In addition, because of taxonomic challenges and conflicts, various proposals have been made to abandon key aspects of the Linnaean system. We review the current status of taxonomy of the living world, focussing on groups at the taxonomic level of phylum and above. We summarize the main arguments against and in favour of abandoning aspects of the Linnaean system. Based on these considerations, we conclude that retaining the Linnaean Natural System provides important advantages. We propose a relatively small number of amendments for extending this system, particularly to include the named rank of world (Latin alternative mundis) formally to include non‐cellular entities (viruses), and the named rank of empire (Latin alternative imperium) to accommodate the depth of diversity in (unicellular) eukaryotes that has been uncovered. We argue that in the case of both the eukaryotic domain and the viruses the cladistic approach intrinsically fails. However, the resulting semi‐cladistic system provides a productive way forward that can help resolve taxonomic challenges. The amendments proposed allow us to: (i) retain named taxonomic levels and the three‐domain system, (ii) improve understanding of the main eukaryotic lineages, and (iii) incorporate viruses into the Natural System. Of note, the proposal described herein is intended to serve as the starting point for a broad scientific discussion regarding the modernization of the Linnaean system.
The localities of Wai Lek and Bang Pu Dam (Krabi Basin, peninsular Thailand) have yielded a rich and diversified mammal fauna whose study led to propose a late Eocene age. Within the fauna, the anthracotheres represent a large part of the fossil remains, as in the Burmese locality of Pondaung. Up to five distinct species of anthracotheres have been recognized and studied. Four of these species are known by their nearly complete skull and mandibles (Siamotherium krabiense, Anthracotherium chaimanei n. sp., Anthracokeryx thailandicus n. sp. and Bothriogenys orientalis n. sp.). The abundance and quality of this material led to define close affinities with Eurasian and African forms and also to put in synonymy the Burmese genus Anthracothema and the Chinese genus Heothema with Anthracotherium. The status of the genus Anthracohyus is also reconsidered by the light of the new primitive taxa from Thailand. In addition, the likelihood of a southern migration of the genera Bothriogenys and Anthracotherium during the Eocene from South Asia respectively to Africa and Europe has been pointed out on the basis of their common occurrence in these areas. Finally, a reappraisal of the origin of Anthracotheriidae and a tentative phylogenetic study of the different Thai species along with the primitive Asian artiodactyls are proposed.
Two fragmentary tooth rows belonging to the small late Eocene anthracotheriid Siamotherium krabiense from Thailand display abnormal morphologies: three lower molars with a neomorphic cusp in place of a paraconid and an upper P4 with only one cusp. Possible causes of such anomalies are reviewed, among which atavism and odontomes can be excluded. Inbreeding or disease are tentatively suggested to have played a role in one of the abnormal morphologies, and an extreme variation is assumed for the second described specimen.
Addition of the recently discovered fossil Nalacetus to a phylogenetic analysis of basicranial, cranial, dental, postcranial, and soft morphological characters reveals that it is the most basal cetacean, and that mesonychians form the monophyletic sister group to Cetacea. The molars of Nalacetus elucidate transformations in dental morphology that occurred early in the cetacean radiation and clarify certain derived differences in molar cusp position between cetaceans and the extinct clade, Mesonychia, hypothesized to be their sister taxon. Nalacetus and other archaic cetaceans share derived vertically elongate shearing facets on the lower molars. Applying the Extant Phylogenetic Bracket, we advance the hypothesis that these facets are an osteological correlate of aquatic predation. Our functional interpretation of this character and its distribution within Cetacea indicates that a behavioral change in tooth use characterized the origin of the clade. Comparison of the transformation of this dental character with that of the cetacean pelvis indicates that a change in tooth use (feeding behavior) occurred before loss of the ability to engage in terrestrial locomotion. The most parsimonious phylogenetic hypothesis presented here has a significant fit with the stratigraphic record as determined by the Manhattan Stratigraphic Measure, which is corroborated by retention indices of stratigraphic data. Ghost lineages necessitated by the phylogenetic hypothesis extend the stratigraphic range of Cetacea into the middle Paleocene (Torrejonian), ten million years earlier than the oldest cetacean fossil currently known. Primitive features of Nalacetus , the large number of synapomorphies diagnosing Cetacea, and the implied ghost lineage suggest that the early cetacean radiation was much more extensive than has been previously recognized.
Cetaceans (whales, dolphins, and porpoises) form one of the most dramatically derived group of mammals and modern representatives are easily recognized by the telescoping of the skull, posterior movement of the narial openings, isolation of the earbones, shortening of the neck, loss of external hind limbs, reduction of the pelvic girdle, and addition of vertebrae (e.g., Barnes, 1984). These skeletal character states are among the most conspicuous features within a suite of transformations that cetaceans experienced in basically all of their biological systems during their adaptation to the aquatic environment.